The present invention relates to a process for the production of at least one nitrogen product having an extremely low level of detectable contaminants and impurities, or an "ultra-pure" nitrogen product.
Variations on traditionally available air separation processes to make high purity nitrogen have been proposed to reduce levels of impurities, such as hydrogen, helium, oxygen, carbon monoxide, hydrocarbons and neon, as concentrations of these constituents in cooled and dried feed air may be as high as 20 ppm. Some of these processes have been successful to reduce impurities in the nitrogen product to low levels.
The semiconductor industry in particular demands levels of contaminants and impurities in process gases to be maintained at an extremely low level, often required to be maintained at or below 10 ppb. Together with the ultra-pure nitrogen requirement, often at the same or nearby facility a gas consumer may have requirements for nitrogen gas of more normal purity, and the relative as well as total volumes required may vary from time to time. These and other factors require new and improved cost-sensitive and flexible processes for separating air into nitrogen products of varying purity, including production of extremely ultra-pure nitrogen.
U.S. Pat. No. 5,218,825 discloses a process for producing both a normal purity and a high purity nitrogen product. Air is compressed, cooled and flowed to a main column operating at or near nitrogen product pressure, wherefrom a nitrogen-enriched stream is withdrawn and a normal purity nitrogen product is taken prior to the nitrogen-enriched stream being increased in pressure and returned to the main column, following expansion, as reflux. According to the process described, a side rectification column takes a feed from the stripping section of the main column and a high purity nitrogen product is produced in the upper portion of the side rectification column. The process utilizes expansion of the oxygen-enriched stream from the bottom of the main column to condense vapors at the top of the main air separation column.
U.S. Pat. No. 5,123,947 discloses a multi-column cryogenic air distillation where ultra-high purity nitrogen, defined as typically less than 0.1 ppm impurities is produced from a nitrogen-rich stream withdrawn from a first column and fed to a second column. The process describes purging a portion of uncondensed vapor produced from the top of a second column, and recovering the ultra-high purity nitrogen product at a point below the purge point in the second column.
U.S. Pat. No. 4,902,321 discloses a process for the production of high purity nitrogen comprising partial condensation of a nitrogen rich vapor stream containing light impurities withdrawn from a main cryogenic air distillation column by indirect heat exchanger with the expanded condensate in a heat exchanger.
In U.S. Pat. No. 5,325,674 a process is disclosed for producing high purity nitrogen comprising expanding a dried and cooled feed air stream into a first air separation column to produce a nitrogen-enriched stream at the top of the column. Also disclosed is the flowing of recycled nitrogen at an elevated pressure through a reboiler located in the lower portion of a second column to provide boil-up, and thereafter flowed into the upper portion of the second column, to produce at the top of the second column vapors containing light impurities which vapors after at least partially condensing in a condenser located in the lower portion of the air separation column, are purged from the second column. High purity nitrogen is produced from the lower portion of the second column.
EP 0 376 465 A1 discloses a method of purifying nitrogen from an air separation process and producing an high purity nitrogen product by charging a nitrogen-enriched stream from a conventional air separation process to the bottom of a column having a reflux condenser. Liquid nitrogen is withdrawn from an upper portion of the column and flashed to generate a liquid and a vapor. The liquid from the flash separation is recovered and flashed a second time to produce the high purity product.
An improved process and installation to effectively carry out the production both ultra-high purity nitrogen and a normal purity nitrogen would be advantageous and is much desired.